Means for making a transducer sound absorbing pad



June 23, 1964 F. E. COYLE 3,137,897

MEANS FOR MAKING A TRANSDUCER SOUND ABSORBING PAD Original Filed May 25, 1962 4 2 Sheets-Sheet l I M iA/ENTOR June 23, 1964 F. E. COYLE 3,137,897

MEANS F OR MAKING A TRANSDUCER SOUND ABSORBING PAD Original Filed May 25, 1962 2 Sheets-Sheet 2 INVENTOR.

CMLLAQUQQ-W flrmfnr United States Patent Ofifice 3,137,897 Patented June 23, 1964 1 Claim. (Cl. 18-165) This invention relates to transducer devices utilizing a sound absorbing pad, and particularly to the apparatus for the sound absorbing pads manufacture.

This is a divisional application of copending application, Serial Number 197,806, filed May 25, 1962.

When electrical energy is applied to a piezoelectric crystal of a transducer, equal amounts of sound energy radiate from each side of the crystal. In a sonar transducer it is often desired to have the energy radiate from one side to obtain directivity and, therefore radiated unwanted encountered energy must be absorbed. A situation where this is encountered is a sonar transducer used at substantial depth to obtain information relative to the wave motion in order to predict the trajectory of a missile fired from an underwater position.

It is, therefore, an object to provide an apparatus for use in producing the sound'absorbing pad.

Other objects will become apparent from the specification and drawings in which the characteristics of the invention are set out.

In the drawing:

FIG. 1 is a plan view of a transducer employing the invention;

FIG. 2 is a cross-sectional view taken along the line 22 of FIG. 1

FIG. 3 is a cross-section through the shock absorbing FIG. 4 is a perspective view of the molding tool, with portions broken away, used to make the pad of FIG. 3;

FIGS. 5 and 6 show the piston and cylinder of the tool of FIG. 4 in inverted position as used in the steps for producing the shock absorbing pad.

The invention is based upon the discovery that high frequency sound waves are absorbed by a batt of loose polyester fibers containing a proper proportion of copper chips distributed through it. A batt of this type formed into a pad and placed to one side of a wave emitting transducer element will absorb the emissions from that side and, consequently, directivity is obtained in that emission is only from the free side.

Referring to FIGS. 1 and 2; 10 indicates a transducer having a hollow cylindrical body 12, externally threaded at 14, and with a bottom or end wall 16. Transducer 10 is adapted to be joined by threads 14 with an unshown stuffing tube extending from an underwater craft; an O- ring 18 in end wall 16 seals the joint between them. Conductors connect the transducer through the stufling tube to the apparatus within the craft. The position of the transducer on the stufling box is an upward looking direction toward the waters surface. The construction of a receiving transducer used in conjunction with the transmitter, for the inventions purposes, is similar and may, if so desired, employ the sound absorbing pad, shown in FIG. 3, to be described hereinafter. Description, however, is limited to a transmitter transducer.

The body 12 of the transducer is generally cylindrical and has an axial cavity 20 opening towards its top. Two apertures, 21 and 22, penetrating the end wall 16 and longitudinal flutes 24 in the peripheral wall defining cavity 20 provide a passageway for leading electric energy to the oscillating element Within the transducer body.

Seated within the transducer body is a co-axial stack of cylindrical elements shaped to fit the bodys cavity 20. The lowermost of these is a coil supporting cartridge 26. Its lower end is externally reduced by an annular groove 28 in order not to cover the end wall apertures 21, and thereby close the passageway between them and flutes 24. The upper end of coil supporting cartridge 26 has a bore 30 and there is a radial groove 32 in the bottom wall 34 defining the bore 36 which receives a coil 36. The combined depth of bore 30 and groove 32 is equal to the height of coil 36. A disk-like coil cover 37 seated on coil support cartridge 26 keeps coil 36 in its desired position. Radial groove 38 in the upper end face of coil cartridge 26 furnishes access for conductors from flutes 24 to coil 36.

A sound absorbing pad 40 fitted into a ring 42, having radial apertures 43, is seated on coil cover 37. Each of the elements, ring 42, coil cover 37 and coil support cartridge 26 has an external longitudinal groove, designated as 35, 35' and 35", respectively, for receiving a dowel 3 mounted in one of the flutes 24. This secures them against rotation within the transducer body cavity 2h.

The piezoelectric oscillator element 48 is a ceramic disk of barium titanate encircled by a spacer ring 50. It is disposed between lower and upper electrical contact rings, 44 and 46, and seated on the sound absorber assembly, ring 42 and pad 40. Contact rings 44 and 46 are provided with external, radial tabs 47 for entering a flute 24 and securing them against rotation. A conductor 51, passing along a flute 24, connects contact ring 44 to a connector 45 in transducer body aperture 21; whereas, a conductor 52 similarly connects, not shown, contact ring 46 through coil 36 to a connector 47 in the other aperture. Connectors 45 and 47 are adapted to seal the apertures against fluid passage and to receive alternating energy from a source, not shown, for transmission of the energy to the oscillator element by the conductors in the flutes.

The elements thus far described are held against axial movement in transducer body 12 by an externally threaded ring nut 54 which is threaded into the bodys cavity 20 and seated on a shim ring 56. Shim 56 is secured against rotation by a tab 57 entering a flute 24.

Transducer body 12 contains oil, not shown, and is closed by a cap assembly 58. The latter includes a rubber disk or sound transmitting window 60 secured at its ends between upper and lower clamping rings, 62 and 64, which are seated on the end face 66 of the transducer body. Rings 62 and 64 are carried in an encircling nut 68 having an inward flange 70 that engages an end face of the upper ring 62 so that they move axially with it. Nut 68 is threaded to the external threads 14 on the transducer body and further secured with a set screw 70. An O-ring 72 under the lower ring 64 in the end face of the transducer body is furnished as a seal against fluid leakage.

In operation, alternating electric energy, applied at the faces of oscillator element 48 through the contact rings, 44 and 46, causes it to vibrate and emit a sound signal in both axial directions of the transducer body 12. The emission from the upper face passes through the oil, the window 60, and the water in which the transducer is immersed toward the Waters surface. An echo results which reflected to suitable equipment for resolution. The energy radiated from the lower face toward the bottom of the transducer body, however, is confronted by the sound absorbing pad 40 and absorbed. Emission is thus in one direction, upward from the transducer. For the purposes set out, oscillation is one megacycle or ten times normal frequency which is more than a standard acoustic material can cope with.

The use of various types of lossy rubber and cork to absorb unwanted acoustical energy that is radiated from transducer elements, may be satisfactory for depths and pressure experienced by submarines of World War II vintage. However, at higher pressures, such as those resulting from depths within the capabilities of present day submarines, these materials lose their lossy properties until this pressure is reduced. In fluid filled transducers, the problem is more severe because the high pressure forces the fluid into the material and then causes a permanent change.

Absorbing pad 40, FIG. 3, which has proven to perform excellently, comprises a fiuffy batt 76 of polyester fibers of ethylene glycol and tereththalic acid, known as Dacron in the art and supplied by the E. I. du Pont Company, having interspersed throughout a multitude of curled copper chips 78. A nylon mesh covering 80 encases it to contain the metal chips and yet allow passage of the transducer fluid. The metal chips are used to disperse acoustical energy. The fibers of the Dacron are not bound to each other and provide a pad which is firm and of stable dimensional form for suspension of the metal chips.

In the production of the sound absorbing pad 40, the copper chips are produced by cutting them from a copper bar by passing a vixen milled curved tooth file (smooth) over it, or else by using a Bridgeport milling machine on the copper bar in which the cutter r.p.m. is 1500, the feed is A inch per minute, and the fly cutter is ground 90 in reference to the work and set at a 3 negative rake. The filings so produced are coarse and have a curl. They are sifted over a 50 gauge screen and the fines discarded.

After preparing the chips are filings, a Dacron batt is taken and pulled apart to fluff it evenly. The copper filings, retained on the screen, in the proportion of 2.8 grams of copper filings per 0.8 gram of Dacron batt (3.5 to 1), are distributed throughout the batt by rolling it in the filings and positioning the filings with tweezers or otherwise until a ball 73 (FIG. 5) is formed.

Converting the ball 73 into the nylon encased pad 40 of FIG. 3 is obtained with use of the mold generally indicated at 82 in FIG. 4. It includes a cylindrical piston 83 the end of which is dimensioned to the size desired in the pad. Piston 83 is threaded externally at its upper end and carries a nut 84; its lower end fits into a sleevelike cylinder 86 which supports it by engagement of its end face with that of nut 84. Adjusting the nut 84 permits raising or lowering the piston 83 in cylinder 86. The piston 83 and cylinder 86 are removably carried on a base plate 88 between the legs of an upstanding yoke 90 secured to the base by bolts 91. The legs of the yoke 90 are notched from their inward sides, 92 and 92', to receive the cylinder 86 with a close fit to eliminate vertical movement of the cylinder. An end pointed bolt 93 is threaded through the bight of yoke 90 and seats into a countersink 94 in the top of piston 83 to restrict vertical movement of the piston.

To form the Dacron ball into the disk shaped and nylon encased pad 40 shown in FIG. 3, the subassembly of the piston 83, nut 84 and cylinder 86 are removed and inverted as shown in FIG. 5. The Dacron ball 73 is placed on the inverted end of piston 83 between disks of nylon, and 95' (FIG. 5), the end of cylinder 86 being level with that of the piston 83. Nut 84 is then turned moving cylinder 86 upward so that its end is now level with the upper nylon disk 95, FIG. 6. The piston cylinder with batt therein is then inverted and seated on the mold base 88 and the adjusting nut turned until the end of the piston within the cylinder 86 is at the desired position above base 88, which is that of the pads thickness, as shown in FIG. 4. Bolt 93 is then turned so that its pointed end seats in countersink 94 thereby locking the mold parts together.

After the mold has been assembled, it is placed on a hot plate and heat applied until the nylon encases the batt. An illustrative temperature and time is 360:5 degrees F. for twenty minutes for a pad having a 1.50 inch diameter and being 0334:.005 inch in thickness.

An absorber pad so produced does not require any glues which are apt to contaminate the oil in the transducer body or impede the acoustic properties by copper chips becoming misplaced as may be the situation in a pad not having the nylon jacket. The pad has been found to be firm, of good dimensional stability, of uniform thickness, and gave excellent acoustic results in its intended environment. The attenuation obtained is not a function of pressure.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

A mold for forming a disk-like pad comprising a base,

an inverted U-shaped yoke secured at the ends of its legs to said base and having opposed notches on the end portions of the inner sides of its legs,

a cylinder having a central aperture therethrough of the desired shape of said pad insertable with horizontal movement between the legs of said yoke into said notches for seating against vertical movement with its lower end on said base,

a piston shaped for entry into said aperture of said cylinder and of sufiicient length to extend through said cylinder,

said piston having external threads at its upper portion,

a nut threaded on said piston adapted to cooperate with the upper end of said cylinder supporting and adjusting the position to said piston in said cylinder, against vertical downward movement,

a bolt in said yoke adapted to engage said piston and securing it against vertical upward movement.

References Cited in the file of this patent UNITED STATES PATENTS 

